The rotation curve of a disc galaxy is a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy's centre. It is typically rendered graphically as a plot, and the data observed from each side of a spiral galaxy are generally asymmetric, so that data from each side are averaged to create the curve. The experimental curves observed are at significant variance with gravitational theory applied to the matter observed in a galaxy. Theories involving unobservable dark matter are the main postulated explanation of this discrepancy.